1. Field of the Invention
The present invention relates to capillary electrophoresis apparatus and pump mechanisms suitable for such electrophoresis apparatus. In particular, the invention relates to an electrophoresis apparatus where a capillary or a capillary array is filled with a gel or a liquid polymer solution which serves as a separating or electrophoretic medium and a pump mechanism suitable for such electrophoresis apparatus.
2. Description of the Related Art
Capillary electrophoresis is in widespread use as a technique to separate/analyze deoxyribonucleic acids (DNAs) and many other biological samples. One of its technical advantages is the superior heat radiation ability due to the capillary's surface area to volume ratio. This heat radiation ability has realized high speed and high resolution separation of analytes by high voltage-used electrophoresis.
Another advantage of the capillary electrophoresis is that analysis can easily be multiplexed by using multiple capillaries at the same time. Multi-capillary high throughput electrophoresis apparatus are put to practical use.
In JP-A-2001-281221 and JP-A-2001-324473, an electrophoresis apparatus using a capillary array composed of 16 capillaries is disclosed. Each capillary is a thin tube with an inner diameter of several ten to several hundred m. It is made mainly of quartz and coated externally with a polyimide film of several ten m in thickness for the purpose of enhancement in mechanical strength. To perform electrophoresis, the capillaries are filled in advance with a component which serves as a sample separation medium.
Although non-liquid cross-linked polymers were used as electrophoretic separation media in the past, it is common in these days to use non-cross-linked liquid polymer solutions since they are superior in productivity and performance stability. JP-A-2001-281221 discloses a pump which fills the capillaries with a gel or polymer as a sample separation medium. This disclosed pump mechanism is a glass syringe. The currently available electrophoresis apparatus include such a one as provided with a pump mechanism which drives a sapphire plunger.
In JP-A-2001-324475, the length or number of capillaries is changed according to the type of analysis and the throughput required by the user. Relatively long capillaries are used in resolution-critical applications where, for example, DNA analytes with several hundred to one thousand bases are separated by the number of bases in steps of one base. Relatively short capillaries are used when resolution is not so critical as analysis speed. Large scale users such as genome analysis centers which deal with many samples demand high throughput while laboratory-level small-scale users may not be dissatisfied with low throughput.
In addition, JP-A-2001-324475 discloses a capillary array which can be replaced by the user. This capillary array is secured at three points, namely its sample introduction end, optical detection section and polymer solution injection end. The polymer solution injection end of the capillary array is connected to the pump mechanism of the electrophoresis apparatus.
Specifically, each of the electrophoresis apparatus disclosed in JP-A-2001-281221, JP-A-2001-324473, and JP-A-2001-324475 uses a 16-capillary array. At the polymer solution injection end, 16 capillaries are tied up in a bundle of about 3.5 mm in diameter. The polymer solution injection end is inserted horizontally into an acrylic block and fixed airtightly by means of a sleeve and locking screws. A reservoir syringe to reserve polymer solution and an injection syringe to inject polymer solution at high pressure into the capillaries are attached to the block. Both syringes communicate with the capillary array through the internal flow passage of the block.
Further, there is provided a second block to which a reservoir to contain buffer solution can be attached. The first block to which the capillary array is attached is connected with this second block by a tube. The capillary array communicates with the buffer solution supplied to the second block via the flow passage formed in the first block, the tube and the flow passage formed in the second block. In addition, since an electrode to serve as an anode is inserted in the buffer solution supplied to the second block, the flow passage from the capillary array to the buffer solution constitutes an electrophoretic passage when a voltage is applied for electrophoresis.
In an electrophoresis apparatus, it is common to inject high viscosity liquid polymer at high pressure into capillaries whose inner diameter is several ten to several hundred microns. If the pressure is low, polymer injection is time-consuming, which lowers the processing capacity of the apparatus. Further, polymer injection is done each time measurement is newly started in order to prevent the variation and deterioration of the performance. To shorten the time required for analysis or raise the processing capacity of the apparatus, a pump mechanism which can stably generate high pressure is needed.
JP-A-2001-281221 discloses a method of utilizing the stalling torque of the motor which generates polymer injection pressure. This method utilizes the DC motor's nature that as the torque grows, the motor decreases its rotation speed and finally stops.
Specifically, the following steps are repeated to control the pressure. (1) If the internal pressure of the syringe rises to a target level, the DC motor stops due to the increased load torque. (2) If the internal pressure of the syringe falls due to progress of polymer penetration into the capillaries, the motor starts rotating. In this case, the current to be supplied to the motor is adjusted so that the motor stops at the target pressure to control the torque characteristics. The target pressure is determined so as not to cause leakage or damage in the flow lines. As well, there exists electrophoresis apparatus where the same pressure control method is implemented by using a stepping motor.